Coral Reef Ecology

Coral reefs are home to an incredibly diverse range of organisms, in many cases living at densities that far exceed those found in other aquatic habitats. This is despite the fact that, in most cases, corals live in waters with low levels of nutrients. The total number of fish species that either live on the coral reefs, or are associated with them at some point during their lives, is estimated at between 6,000 and 8,000. Put another way, that is well over one-third of all the known fish species in the world in a habitat that occupies less than 1% of the world's seas.

Apart from fish, what other types of animal are found on coral reefs?

Coral reefs are also home to a bewildering number of invertebrate species, including crustaceans (such as shrimps, lobsters, and crabs), echinoderms (starfish and sea urchins, for example), sponges, worms, mollusks, and sea squirts, to name but a few. There are even thousands of species of coral itself. Of the 34 different animal phyla on Earth (each phylum is a taxonomically distinct group), 32 occur on coral reefs. When compared with rainforests, where nine, or at the most 10, can be found, it is easy to see why biologists consider coral reefs to be unique.

Why are coral reefs so diverse?

This is one of the biggest questions in ecology and several answers have been proposed. One popular current theory holds that the very high levels of predation experienced by all reef animals reduces the probability of any species becoming dominant on the reef through its ability to out-compete others. But although it is difficult to say exactly what their relative importance is, it seems very likely that high productivity, high competition, and high predation have all played a significant role in the evolution of so many different species on coral reefs.

Another facet of reef communities is that they are actually quite robust. Among the millions of individuals representing thousands of species found on the average reef, large population cycles and fluctuations happen year after year, yet the community remains working and intact. One reason for this is that if a species has a bad year — caused, for example, by poor recruitment of juveniles to the reef following their larval drifting stage, or by relentless competition or predation — there is always another species available to fulfill its ecological role. So if one grazing herbivore becomes locally rare, there are plenty of others ready to step in.

What different habitats can be found in the tropical marine ecosystem?

Many coastal reefs have very distinct zones — the sheltered lagoon is protected from the battering of the waves and here seagrass meadows form in the warm shallow waters. Delicate corals are better able to grow on the lagoon side of the reef, again because of the protection from the waves.

Heading in towards the shore from the fore reef, the shallow reef flats are bathed in sunshine. Here, numerous algae species use the sun to photosynthesize and are in turn grazed by fish and invertebrates. Sometimes this reef flat stretches for hundreds of feet and can be exposed at low tide. Occasionally, there may be breaks and channels in the reef, allowing direct communication between the lagoon and the open sea. These areas are often associated with strong water currents and can be a favorite haunt of filter-feeders and planktivorous fish.

The seaward face of the fore reef descends from the crest near the ocean surface down to the sea floor. The upper parts of the reef face are well supplied with nutrients and food, delivered by the ocean currents, but are also exposed to the waves. Only the toughest corals can survive here.